Passively mode-coupled femtosecond laser

ABSTRACT

The invention relates to a passive mode locked femtosecond laser whose ring resonator comprises the elements: a) a laser-active element  1 , b) at least a dielectric mirror  2  having a negative group velocity dispersion GVD such that for a continuous part of the optical spectrum amplifieable by the laser-active element the sum of the group velocity dispersion of the mirror  2  and the positive group velocity dispersion of the laser-active element  1  is negative, i.e.            ∑   n          GVD   n       &lt;   0                   
     c) two concave mirrors  21  and  22  which are spatial adjacent next to the laser-active element and which are orientated with their concave surfaces towards the laser-active element  1  and d) an optical output coupler  3 . The optical path length in the resonator is below 60 cm, advantageously below 30 cm, in particular below 15 cm, the focus length f 21  and f 22  of the concave mirrors  21  and  22  are below 3 cm, advantageously below 2 cm, in particular below or equal to 1,5 cm, such that a high repetition generation of femtosecond pulses is achievable. 
     Further the invention relates to a continuous tuneable high repetition rate femtosecond laser, the method of use thereof for high repetition generation of femtosecond pulses and a method for tuning thereof.

The invention regards a passive mode-locked femtosecond laser having aring resonator comprising a laser-active element, an optical outputcoupler and at least one mirroring element. Further the inventionregards a femtosecond laser according the preamble part of claim 7 andstill further a femtosecond laser according to the preamble part ofclaim 1. Further the invention regards a method for tuning a femtosecondlaser. Also the invention regards a method of use of a femtosecond laserfor generation of laser pulses with a duration below one picosecond.

With passive mode-locked femtosecond laser systems relatively high pulsrepetition rates in the range of several hundred MHz may be reached.These laser systems nevertheless cannot be referred to as being of highrepetition rate as their repetition rates do not exceed 500 MHz. Due toextremely low pulse durations the pulses have a high peak intensity.Femtosecond lasers nowadays are successfully employed in the field oftime-resolved spectroscopy, non-linear optics,multiple-photon-microscopy, micro-material engineering, opticalfrequency metrology and optical coherence-tomatography. Further in thefuture such lasers will also play a substantial role in the field ofoptical data communication.

Conventional passive mode-locked femtosecond laser systems rely on theuse of titan doped sapphire crystals as laser-active elements. Uponoptical excitation these develope a broad fluorescence spectrum in therange between 700 and 1000 nm. From this a gain profile of comparablerange results, which means that Ti:Sapphire laser systems are suitablefor generation of laser light in said range of wavelengths.

For generation of ultra-short laser pulses it must be observed that alaser pulse which is short in the time domain is correlated with a broadfrequency spectrum. Because of this reason only laser elements with abroad gain profile are suitable for generation of ultra-short pulses.

All yet known passive mode-locked femtosecond laser systems with a solidstate laser-active element (a CPM-dye laser is also passive mode-locked)rely on the concept of “Kerr-lens mode-locking”. This non-linear opticaleffect results, by self-focussing of an intensive light beam in anon-linear medium, in a temporarily gain of a single pulse in thelaser-active element during its round trip in the resonator, compared toa continuous operation of the laser.

The repetition rate of such a femtosecond laser system is determined bythe duration of a round trip of the pulse circulating in the resonator.

The duration of the pulse circulating in the resonator is neverthelessnot able to reach the theoretical limit, which is determined by thewidth of the gain profile of the laser-active elements. This is causedby the phenomenon of pulse-broadening, which is experienced by the laserpulse in particular in the laser-active element during its roundtrip inthe resonator. This effect is due to the so-called positive groupvelocity dispersion of the laser-active elements and further opticalcomponents in the resonator. The consequence is, that the variousportions of wavelengths of the circulating pulse pass the laser-activeelement within varying time periods, whereby the laser pulse passingthrough the laser-active element is broadened in its time duration.

The basic approach to compensate pulse broadening which is caused by thepositive group velocity dispersion of the laser-active elements andfurther optical components of the resonator is the use of an arrangementin the laser resonator comprising a negative group velocity dispersionwhich at least compensates the pulse broadening which has been caused bythe laser-active element and the further optical components.

Known in prior art is for instance an arrangement of two dispersiveelements, for instance prisms in a laser resonator, a so-called prismcompensator.

Basics of femtosecond laser systems which rely on “Kerr-lensmode-locked” Ti:Sapphire lasers with prism compensators can for instancebe drawn from the publication of D. E. Spence, P. N. Kean, W. Sibbet inOptics Letters 16, page 42 and following pages (1991).

Recently as an alternative to said prisms or prism compensatorsdielectric mirrors have been developed which provide a negative groupvelocity dispersion GVD. This is achieved by a suitable sequence ofdielectric layers on a substrate. The basic concept can be drawn fromthe publication of R. Szipöcs, K. Ferencz, Ch. Spielmann, F. Krausz inOptics Letters 19, page 201 and following pages (1994).

The use of such mirrors with negative group velocity dispersion GVD in alaser resonator offers a substantial advantage, i.e. in contrast to theabove mentioned prisms- or prism compensators only a non-significantprolongation of the optical path in the resonator occurs.

A femtosecond laser system, which is based on said mirrors, may forinstance be drawn from the publication of H. Stingl, Ch. Spielmann, R.Szipöcs, F. Krausz in Conference on Lasers and Electro-Optics 9, 1996OSA Technical Digest Series (O.S.A., Washington D.C., 1996) page 66 andfollowing.

Most of the Ti:Sapphire femtosecond laser systems relying on thephenomenon of “Kerr-lens mode-locking” comprise a Fabry-Perot-resonator,whose markable feature is a planar end mirror and which in particularhas a folded configuration. In such kind of configuration elementsbelonging to a pulse compression such as prisms may be allocated simplyin one arm of the resonator. The total length of such a resonatoramounts typically in the range of 2 meters. Therefrom typical pulserepetition rates in the range of a few megahertz of normally below 100MHz result. Such laser systems are not labelled as having ahigh-repetition rate.

From the above-mentioned publication of A. Stingl et. al. for instance apassive mode-locked Ti:Sapphire femtosecond laser system is known, whichrelies on a Fabry-Perot-resonator and which makes use of mirrors havinga negative group velocity dispersion GVD.

Further from the U.S. Pat. No. 5,383,198 a self-starting passivemode-locked femtosecond laser system is known, having a prism compressorand a ring resonator and also from the U.S. Pat. No. 5,799,025 aself-starting passive mode-locked femtosecond laser system is known,having a prism compressor and a Fabry-Perot-resonator.

Due to the respective resonator geometries none of the mentioned lasersystems allows to achieve pulse repetition rates of above 500 MHz andtherefore these laser systems cannot be labelled as having highrepetition rates.

An alternative approach may be drawn from the publication of M.Ramaswamy and J. G. Fujimoto in Optics Letters 19, page 1756 andfollowing pages (1994) (see also U.S. Pat. No. 5,553,093). The approachis based on a simplified resonator configuration with making use of aspecific prism compressor. Instead of a conventional resonator-internalpair of prisms a prism-shaped laser crystal and a prism-shaped outputcoupler is used. A specific geometry of the laser resonator configuratedas a Fabry-Perot-resonator and also the simplified prism compressorallows a shortening of the resonator length to about 30 cm so that arepetition rate of 1 GHz may be achieved.

Disadvantageous on this concept is, that in the outgoing beam thevarious in a laser pulse superimposed spectral components spread apartin a spatial direction perpendicular to the direction of the laser beam(“spatial chirp”), which at least complicates a practical use of suchlaser concept.

In fact the prism compressor being located inside the resonator enforcesa minimal length of the resonator upon which it appears unlikely andeven impossible that higher repetition rates than 1 GHz may be achieved.

A prism compressors causes, additionally to a negative group velocitydispersion, in some parts of the resonator a spatial split-up of thevarious spectral components of a laser pulse circulating in theresonator. A selection of wavelength may be established by use of asuitable, re-allocatable aperture in such area of a resonator, in whichthe spectral components are spatially split up. For such accomplishedselection of wavelength the resonator facilitates laser activity.Thereby an ability of wavelength tuning is achieved. From thefemtosecond laser system “Tsunami” of the Spectra Physics Company,Mountain View, Calif. such kind of arrangement is known for instance.

However, due to the respective resonator geometries further with none ofthe mentioned laser systems it is possible to achieve pulse repetitionrates exceeding 500 MHz may be achieved and simultaneously the abilityof wavelength tuning. Therefore, these laser systems cannot be labelledas being of high repetition rate.

It is therefore an object of the invention to provide a passive modelocked high repetition rate femtosecond laser by which laser pulseshaving a duration below one picosecond can be generated and which at thesame time can be operated conveniently at repetition rates above 500MHz, in particular above 1 GHz. A laser system according to a firstvariant of the invention shall in particular be used to generate suchlaser pulses.

Furthermore such laser system shall be adapted to generate therewithcontinuous tuneable laser pulses within the range of the gain profile ofthe laser-active medium and with a duration below one picosecond andwhich laser system is at the same time conveniently operable atrepetition rates of above 500 MHz, in particular of above 1 GHz. A lasersystem according to a second variant of the invention shall inparticular be used to generate such laser pulses.

In such laser systems the laser beam coupled out from the laserresonator shall not suffer from a spatial spreading of a spectralcomponent perpendicular to the direction of the laser beam.

Furthermore a required occupied area of such a laser system shall besignificantly reduced compared with laser systems of prior art, inparticular compared with commercial available laser systems.

Additionally an especial suitable method for tuning a high repetitionrate laser system shall be provided.

To solve such object the invention in a first variant proceeds from apassive mode locked femtosecond laser according to the preamble part ofclaim 7.

Such a femtosecond laser comprises a laser-active element which islocated between the concave surfaces of two concave mirrors. Theresonator may also comprise further mirroring or other optical elements,for instance planar mirrors. A respective ring resonator comprisesadditionally at least one dielectric mirror, i.e. one or a number ofthem. A dielectric mirror has a negative group velocity dispersion GVD.The negative group velocity dispersion GVD is adapted such that for acontiguous portion of the optical spectral range which is capable ofbeing amplified by the laser-active element, the sum of the negativegroup velocity dispersion GVD of the dielectric mirror (or, if so, ofthe number of dielectric mirrors) and the positive group velocitydispersion GVD of the laser-active element (and, if so, of furtheroptical elements), is negative. This means${\sum\limits_{n}{GVD}_{n}} < 0$

This is the basic assumption for a generation of femtosecond laserpulses. Further an optical output coupler is arranged in the resonator.

According to a first variant of the invention such a resonator isconstrued such that the optical path length in the resonator is below 60cm, in particular below 30 cm, in particular below 15 cm. From theseoptical path length pulse repetition rates result which convenientlyexceed 500 MHz, which in particular exceed 1 GHz and which in particularexceed 2 GHz. Furthermore the focus length of the concave mirrors, whichare spatial adjacent next to the laser-active element are elected to bebelow 3 cm, in particular below 2 cm, in particular below or equal to1,5 cm elected. With such a laser a high repetition rate pulsed laseroperation with femtosecond pulses is achievable so that said femtosecondlaser is a high repetition rate femtosecond laser.

A passive mode locked high repetition rate femtosecond laser comprisingthese features provides a series of substantial advantages. Making useof a ring resonator instead of a linear Fabry-Perot resonator allows asignificant reduction of the resonator length, which is a basicassumption to be able to achieve high repetition rates.

Making use of dielectric mirrors with negative group velocity dispersionGVD instead of conventional prism- or grating-compressors allowsadditionally to reduce the length of the ring resonator. Thereby it ispossible to reduce the geometric length of the resonator by more than 10cm, which in turn is accompanied by an increase of the pulse repetitionrate.

Furthermore the invention has arisen from the idea that making use ofconcave mirrors, which are especially arranged adjacent next to thelaser-active element and whose focus length amount within the mentionedpredetermined range according to the characterising part of claim 7,allows to achieve a beam diameter of a resonator mode in thelaser-active element, which is comparable to the evolution of theresonator mode of a conventional linear Fabry-Perot- or also of aring-resonator each having a significant larger geometric length. Such asmall diameter of the resonator mode in the laser-active element, inparticular of a beam waist in the laser-active element is a basiccondition for an efficient generation of laser pulses. In particularthis is a basic condition for both of the mentioned fundamental effects,which are responsible for the generation of ultra short pulses in apassive mode locked laser system. These fundamental effects are theso-called self-phase-modulation SPM and the so-calledself-amplitude-modulation SAM in a non-linear medium, in particular alsoin a laser-active medium such as the laser-active element in aresonator. Both effects in turn are due to the so called Kerr-effect.The dependence of the index of refraction from the local light intensityis referred to as Kerr-effect.

The self-phase-modulation SPM occurring in the laser crystal ischaracterised by the quantity Φ and is substantial for the minimalachievable pulse length τ. The relation holds:${\tau\infty}\frac{D}{\Phi \quad E_{p}}$

Therein D is the group velocity dispersion GVD summarised for alloptical elements in the resonator: $D = {\sum\limits_{n}{GVD}_{n}}$

E_(p) refers to the pulse energy.

The magnitude of the self-phase-modulation Φ is essentially proportionalto the square of the beam radius w in the laser crystal. To aim for apreferably high intensity in the laser crystal it is thereforeinevitable to achieve a preferably small beam diameter of the resonatormode in the laser-active element, in particular a beam waist withpreferably small diameter in the laser-active element. This specificeffect is achieved according to the invention by making use of a convexmirror having a focus length adapted to the above mentioned maximalresonator length.

Thereby the further advantage is achieved, that the laser beam coupledout of the laser resonator is free from a spatial spreading of thespectral components perpendicular to the beam direction.

Also the required occupied area of the proposed laser system issignificantly reduced compared to known laser systems of prior art, inparticular to commercially available laser systems, which is achieved bythe skilful choice of resonator geometry.

Further to solve the object, the invention in a second variant proceedsfrom a passive mode locked femtosecond laser according to the preamblepart of claim 1. This in particular regards the generation ofcontinuously tuneable laser pulses within the gain profile of alaser-active medium.

Such a femtosecond laser according to the second variant of theinvention comprises like the first variant of the invention, alaser-active element, which is located between the concave surfaces oftwo concave mirrors. Additionally to the further features of the ringresonator as mentioned with the first variant (at least a dielectricmirror, two concave mirrors an output coupler), the second variantcomprises a self-focussing element and a prismatic element.

According to the second variant of the invention said resonator isconfigured such that the optical path length in the resonator is smallerthan 60 cm, advantageously smaller than 30 cm, in particular smallerthan 15 cm. From these optical path lengths pulse repetition ratesresult, which conveniently exceed 500 MHz, in particular exceed 1 GHz,which are in particular higher than 2 GHz. Further the focus length ofthe concave mirrors, which are arranged adjacent next to thelaser-active element, are selected to be below 3 cm, advantageouslybelow 2 cm, in particular below or equal to 1,5 cm. Furthermore, atleast the dielectric mirror or the output coupler or any further mirroris tiltable such that, due to a tilted angle, in correlation with thespatial dispersion of the prismatic elements, a wavelength isadvantageously continuously tuneable for which wavelength the resonatorallows laser operation.

With such a laser also a high repetition rate laser pulsed operation offemtosecond pulses is attainable so that such laser is a high repetitionrate femtosecond laser. Furthermore, the proposed high repetition ratefemtosecond laser is continuously tuneable. The resonator comprises aprismatic element, which functions to effectuate a spatial separation ofdifferent spectral components of the light circulating in the resonator.According to its second variant the invention arises from the idea, thatin correlation with a mirror tiltable upon an axis perpendicular to theplane in which the spectral components are separated, a wavelength isselectable by said tilt such that for said wavelength the highestoverall gain is supplied in the resonator. This wavelength thereon isthe operating wavelength of the laser.

Although a tuneability may be achieved with most femtosecond lasersystems only with some expenditure and will mostly as a general rule beachieved only to the expense of pulse duration and repetition rate (e.g.by making use of a prism compressor, which is room-expensive andsupports significantly the broadening of pulses) it is nevertheless inparticular advantageous to provide a wavelength-tuneable femtosecondlaser. This is because such laser allows to adapt the optical wavelengthwithin the range of the underlying gain profile to the requirements of adesired application. This has been succeeded with the femtosecond laseraccording to the second variant of the invention in an especial elegantand simple way. This is because the proposed continuous tuneable highrepetition femtosecond laser comprises also the advantages of the laseraccording to the first variant of the invention. In particular theability of high repetition, compactness, tuneability and femtosecondoperation mode have been realised in combination.

Thereby it is advantageous, according to a further developedconfiguration of the in particular second variant of the invention, thatthe laser-active element also functions as a self-focussing element,i.e. the laser-active element and the self-focussing element areidentical. For instance this is the case for titan sapphire lasers.

Advantageously the resonator is, in correlation with the non-linearelement, configured such that pulsed laser operation allows a higherdegree of energy efficiency from the laser-active element than in acontinuous operation mode.

The prismatic element is advantageously configured such that an opticalbeam incident on a prism surface under the condition of minimaldeflection, is incident in the Brewster angle with a wavelengthamplifiable by the laser-active medium and also emerges under such anglefrom the output surface.

Optional such prismatic element may be also configured such that anoptical beam incident on the prism surface with the Brewster angle witha wavelength amplifiable by the laser-active medium emerges from theoutput surface essentialy in a right angle. Therefore an anti-reflexlayer is applied to the output surface, the anti-reflex layer beingadapted for those wavelengths amplifiable by the laser-active medium.The above mentioned further developed configurations of the inventionmay be in particular applied to the second variant of the invention andalso to the first variant of the invention.

In particular it is to be observed upon the further developedconfigurations of the invention according to its first variant and alsoto its second variant that all further parameters of the resonator areselected such that an optical stable resonator is being configured.

It has been shown that an especial advantageous laser system isavailable if the focus length of the concave mirrors which are spatialadjacent next to the laser-active element are elected essentially inequal way. This is enabled mainly by an essentially axial symmetricformation of the laser resonator.

In a further preferred development of the laser system according to theinvention the geometric distance of the concave mirrors to each other isselected smaller than the sum of the focus lengths of the concavemirrors. By solely varying the distance d of the concave mirrors to eachother while keeping the total remaining resonator geometry constant oneis able to determine an interval of this distance within which a stableoperation of the laser resonator is possible. Such interval is arrangedessentially symmetric to a distance of the concave mirrors whichdistance corresponds to accurately the sum of the focus length of theconcave mirrors.

It has become apparent that an especial stable operation in a passivemode locked condition is possible if the distance d of the concavemirrors to each other is selected smaller than the sum of the focuslength of the concave mirrors. A positive effect on the evolution of theKerr-effects substantial for passive mode locking results.

This effect can even be enforced if the laser-active element is arrangedbetween the adjacent concave mirrors not in a symmetric but as definedaccording to the features of claim 10. If such arrangement of thelaser-active element is realised a decrease of the beam diameter in thelaser crystal along with an increasing instantaneous power of the pulseresults. This effect is also based on the Kerr-effect and is referred toas a formation of a “smooth aperture” if it is in combination with astronger focussing of the pump-laser beam as compared to the resonatormode. It also supports a stable passive mode locked pulsed operation.

In a further developed configuration of the laser system according tothe invention the distance d between the concave mirrors is selectedlarger than the sum of the focus length of the concave mirrors. In thiscase the occurrence of a “smooth aperture” of the laser-active elementmay not be observed. However, instead of this a so-called “hardaperture” may be located in the ring resonator, which for instance maybe configured as a ring aperture. In particular such a hard aperture isprovided at such location in the resonator on which the resonator modehas a comparingly small diameter with a comparingly large instantaneouspower.

It has become apparent that a laser system according to the invention isadvantageously used with a titan doped sapphire crystal as alaser-active element. Nevertheless further also the use of otherlaser-active elements is possible which comprise a gain spectrum broadenough to theoretically enable the generation of femtosecond laserpulses. In particular the laser-active elements of claim 12 are to bementioned in this context.

To provide optical pumping to the laser-active element for instance anargon ion laser may be used which in particular is tuned to the maximumabsorption of the laser-active element.

In an especially advantageous developed configuration of the lasersystem according to the invention a solid state laser system is employedas a pump laser whose wavelength spectrum is selected to be adapted tothe absorption spectrum of the laser-active element. In particular thefrequency doubled solid state laser systems as mentioned in claim 13 aresuitable in this context. The use of a naturally low-noise solid statelaser system as a pump laser effectuates in an advantageous way thefluctuation of intensity of the pulsed laser radiation generated fromthe inventive laser system.

Further it has become apparent that to support a stable passive modelocated pulsed operation with a comparingly well degree of efficiency ofthe inventive laser system the degree of outcoupling T of the opticaloutput coupler should be below 5%, advantageously below 3%, inparticular below or equal to 2%. Higher degrees of outcoupling T wouldincrease the quantum efficiency of the laser system, smaller degrees ofoutcoupling T would stabilise the inventive laser system in the passivemode locked pulsed operation mode. The output coupler is advantageouslyconfigured as a partial-reflecting, in particular, as a dielectricmirror.

Alternatively also other possibilities for outcoupling may be realised,for instance by outcoupling of an evanecent wave which results from aninner total reflection of a resonator mode on an interface. In such aconfiguration a degree of outcoupling may be varied freely withinspecific bounds.

In a further developed configuration of the inventive laser system allmirrors of the ring resonator have a negative group velocity dispersionGVD. Respectively, depending on the dimension of the positive groupvelocity dispersion GVD due to the laser-active element, also only asingle mirror with negative group velocity dispersion GVD may beemployed in the laser resonator. In particular also the outcouplingmirror may have a negative group velocity dispersion GVD.

In a particular advantageous developed configuration of the inventivelaser system the ring resonator is construed, such that the resonator iscompensated for astigmatism. This may be realised by a suitable choiceof the resonator geometry, in particular of the angle of reflection ofthe concave mirrors. In particular a beam waist with circularcross-section in the laser-active element and/or a beam coupled out fromthe laser resonator having a circular cross-section may be realised inthis way. It is especially advantageous for compensation of astigmatism,when the optical length of the ring resonator is selected to exceed 1cm, advantageously to exceed 2 cm and is selected in particular above3,5 cm. This is because at smaller wavelengths extremely high angles ofreflection at the concave mirrors are to be achieved for compensation ofthe astigmatism of the laser-active element.

Particular practical advantages by making use of the inventive lasersystem result when all elements of the ring resonator are arrangedmechanically on a common mounting platform. In particular a mounting ofthe optical components in a monolithic block is possible.

Particular advantages of the inventive laser system thus are based onits high pulse repetition rate which may extend between 500 MHz andabout 10 GHz. Further advantageous is its significant reduced demand foroccupied expanse, i.e. occupied space, compared to conventional lasersystems. Saving of costy room on optical tables is accomplished. Finallythe signal to noise ratio relating fluctuation of intensity is improvedby more than a factor of 10 compared to conventional, in particular gaslaser pumped laser systems. Further the advantage is achieved, that thelaser beam coupled out of the resonator does not show any spreading ofspectral components perpendicular to the beam direction. Even further avariant of the invention allows for the ability of continuous tuning ofthe inventive laser system.

Further features and advantages of the inventive laser system areillustrated along with the following preferred embodiments, which arenot meant to be restrictive and which are illustrated along with thedrawing. In the drawing it is shown in:

FIG. 1 the evolution of the radius w of the beam waist in a resonator ofconventional length compared to a conventional shortened resonator whichis adapted for a pulse repetition rate of 1 GHz,

FIG. 2 the evolution of the radius w of the beam waist in a laser-activeand simultaneously self-focussing element as a function of the resonatorlength for a ring resonator the geometry of which is optimised to aresonator length of two meters,

FIG. 3 a schematic view of a laser system according to a first variantof the invention in a top view, in which the ring resonator isestablished with two concave mirrors and four planar mirrors,

FIG. 4 a schematic illustration of a laser system according to a firstvariant of the invention whose ring resonator is established with twoconcave mirrors and two planar mirrors in a top view and

FIG. 5 a further development of the laser system shown in FIG. 3 forestablishing of a self start function in a predetermined direction ofcirculation,

FIG. 6 a schematic view of a wavelength tuneable laser system accordingto a second variant of the invention comprising a Brewster prism,

FIG. 7 a schematic view of a wavelength tuneable laser system accordingto a second variant of the invention comprising half a Brewster prism,

FIG. 8 a schematic illustration of the relative position X1, X2, d ofconcave mirrors and a laser-active self-focussing element,

FIG. 9 the progress of stability of an inventive laser whose resonatoris adapted for a pulse repetition rate of 1 GHz as a function of themirror distance d,

FIG. 10 a measured beam cross section of a pulsed laser beam coupled outfrom the inventive laser system also comprising a Gauβ function fittedthereto and as well in the insert an evolution of intensity measuredperpendicular to the beam direction with a CCD camera,

FIG. 11 the evolution of a beam radius w behind the output couplermirror in the plane of the laser resonator as a function of the distancefrom the output coupler mirror in a laser system according to theinvention,

FIG. 12 the evolution of the output power as a function of the pumppower for an inventive laser system having a resonator in pulsedoperation with a repetition rate of 1 GHz for two varying degrees ofoutcoupling T.

FIG. 1 shows the progression of a resonator mode in a conventionalprolongated resonator having a resonator length of two meters as afunction of the position of the beam axis in the resonator as it is inthe region of the laser crystal. As a comparison the progression of aresonator mode in a conventional resonator is shown whose length wasoptimized to a pulse repetition rate of 1 GHz, however, without adaptingthe geometry of the resonator. The inset points out that a naiveshortening of the resonator length of a conventional prolongatedresonator may lead in principle to an increase of the pulse repetitionrate, however, simultaneously (and unavoidably) to an increase of thebeam waist diameter of the resonator mode in the laser-active element.This effectuates the Kerr-effect in a negative way, which is elementarfor the generation of pulses, in particular, the formation of aKerr-lense in the laser-active element is effected in a negative way. Asimple truncation of the resonator length of a conventional elongatedresonator to a length necessary for high pulse repetition rates willtherefore in general lead to the result that no pulse operation of thelaser may be achieved at all or at least no stable pulse operation maybe achieved. Therefore, modifications of the resonator geometry areindispensable for transition to higher pulse repetition rates.

The above issue is once more clarified in a different way in FIG. 2.Namely it is impossible to shorten the resonator length of an alreadyexisting ring resonator with lower pulse repetition rate to a respectiveshorter length to increase the pulse repetition rate. The radius of thebeam waist in such a conventional ring resonator of prior art isillustrated in the laser-active/self-focussing element 1, 2 as afunction of the total length of the ring resonator. A strong increase ofthe beam waist radius is recognised when the total length of theresonator is shortened to a length of about 30 cm as necessary for apulse repetition rate of 1 GHz. Such an increase of the beam waistradius effectuates in an extremely disadvantageous way the progressionof the Kerr-effect in the laser-active element and therefore a stablepassive mode locked operation of the laser system.

To achieve higher pulse repetition rates use is made of reduced focuslengths f41, f42 and accordingly f21, f22, of the concave mirrors 41, 42and accordingly 21, 22, as compared to known laser systems of prior art,and as defined according to claim 1 or claim 7 of the laser systemaccording to the second and first variant of the invention which is ofdecisive relevance.

A preferred embodiment of the laser system according to a first variantof the invention can be drawn from FIG. 3 as follows.

FIG. 3 illustrates a schematic view of a laser system according to afirst variant of the invention in a top view. The laser resonator isestablished by the concave mirror 21 and 22 as well as the planar mirror2 and the planar output coupler 3. The laser-active element 1 is of atitan doped saphire crystal having an absorption coefficient of A=5 percm at the pump wavelength of 532 nm. The surfaces of the crystal asextending in the optical path are planar-parallel and optical polished;their distance amounts to 2,2 mm. The focus length of the concave mirror21 and 22 as in use amounts to 15 mm. The distance d between the concavemirrors is smaller or equal to 30 mm. The titan-saphire crystal islocated approximately midway between the concave mirrors, in particularaccording to the features of claim 4. A frequency doubled Nd:YAG-laserhaving a wavelength of 532 nm is used as a pump laser. By a lense L thisis focussed beyond the concave mirror 21 into the laser-active element1. Thereby the resonator mode and the pump laser beam are superimposedin the laser-active elements as accurate as possible. The angle enclosedbetween the beams reflected by the concave mirrors 21 and 22 amountbetween 15° and 25°, in particular 18°. The total length of theresonator is selected such that a pulse repetition rate of 1 GHz arises.The planar mirror CM wears a high reflective dielectric coating having anegative group velocity dispersion. The also planar output coupler OC isconstrued as a partial-reflective dielectric mirror. Its reflectivityamounts between 95 and 99%, in particular 98%. Advantageously, howevernot necessarily, behind the output coupler OC a photodiode PD islocated. Such a photodiode PD allows, by an intensity measurement, toverify a pulsed operation of the illustrated laser system in a mostsimple way, namely by failure of appearance of a light intensity on thephotodiode. FIG. 4 shows a laser according to a first variant of theinvention as already illustrated in FIG. 3. Compared therewith, thisembodiment comprises two mirrors 4 less. Such a formation is inparticular advantageous to obtain high pulse repetition rates as thereduced number of mirrors allows to achieve a shortened total resonatorlength.

A laser system according to the invention may therefore also be realizedwith a smaller number of mirrors compared to the number of mirrors inthe system shown in FIG. 3. Such a system like in FIG. 4 relies only onmaking use of only two concave mirrors and two planar mirrors. In mostparts it corresponds to the system shown in FIG. 3. However, only theplanar mirror labelled as N1 is implemented as a dielectric mirror 2having a negative group velocity dispersion GVD. All further mirrorswear a conventional dielectric coating which is implemented on theconcave mirrors as a high reflective coating as well as on the outputcoupler labelled as OC with a transmission T of approximately 2%. Due tothe fact that only one mirror having a negative group velocitydispersion GVD for compensating the positive group velocity dispersionof the laser-active element is available it is advantageous to keep thepositive group velocity dispersion of the laser-active element as low aspossible. This can for instance be realized by a reduced length of thelaser-active element. This has been achieved in the laser system shownin FIG. 4 according to a first variant of the invention wherein thedistance between the planar and parallel surfaces of the titan saphirecrystal amounts to only 1,3 mm in distinction to the example shown inFIG. 3. The surfaces of the titan saphire crystal are inclined to theoptical axis with the brewster angle. To achieve a comparable effectiveabsorption of the pump light within the reduced length of thelaser-active element 1 it is advantageous to dope the laser-activeelement 1 with a higher concentration of laser-active ions or atoms. Inthe case illustrated the titan doping has been increased such that theabsorption coefficient of the laser-active element 1 rises to an amountof A=6 per cm at the pump wavelength. In this way a comparableabsorption of the pump light in the laser-active element can be achievedcomparable to those in the laser system shown in FIG. 3.

FIG. 5 illustrates an embodiment of an advantageous development of afemtosecond laser according to a first variant of the invention—forinstance being constitutive on the embodiment of a laser systemaccording to a first variant of the invention as illustrated in FIG. 3.This regards predominantly the additional attached planar mirror M6. Aslong the laser system according to the invention acts in the continuousoperation mode two counter propagating partial beams circulate in thering resonator. Thereby, two partial beams are coupled out of theout-couple mirror 3. The planar mirror M6 serves to reflect one of thesepartial beams back into the resonator. This has the effect, that theintensity of the circulating partial beam circulating in the resonatorin clockwise direction is increased in the laser-active element 1.Therefore, this partial beam is favoured upon laser stimulation andexperiences a stronger Kerr-lens formation in the laser-active elementso that a pulsed operation of the inventive laser system with a pulsecirculating in the ring resonator in clockwise direction is favoured.The mirror M6 therefore serves to predetermine the direction ofcirculation of the laser pulse in the ring resonator.

Additionally, the planar mirror M6 effectuates that the femtosecondlaser according to a first variant of the invention experiences aspontaneous transition from an initial continuos laser operation into apulsed operation. This effect may even be fortified by applying aperiodical perturbation to the mirror M6, for instance by a periodicalvibration.

With a femtosecond laser according to the invention as described in theembodiments of FIGS. 3, 4 and 5 the ring resonator thereof has beenoptimized to a pulse repetition rate of 1 GHz according to theinvention. Thereby, it became possible to achieve an average laser powerof 100 Milliwatt (mW) with the pump power of 1,7 Watt whereby thegenerated pulses have a duration of approximately 50 femtoseconds. Foran inventive femtosecond laser/laser system, whose ring resonator hasbeen optimized to a pulse repetition rate of 2 GHz it was possible togenerate pulses of approximately 25 femtoseconds duration. A slightdependence of the pulse duration X on the intensity of the pump laserbecame apparent. At an average pump power of 2,5 Watt the average pulseduration τ amounts to 29 femtoseconds; at an average pump power of 5,5Watt the average pulse duration τ decreases to 25 femtoseconds. Ameasurement of the wavelength spectrum of the laser during pulsedoperation revealed that the width of the measured wavelengthdistribution agreed well with high accuracy with the measured pulseduration. This means that the pulse duration is limited primarily bydispersive effects of third order.

With the transmission of the out coupled beam through the substrate ofthe out couple mirror 3, which is typically of the thickness of 5 mm,also a positive group velocity dispersion occurs, which results in anextension of the pulse duration. Such extension of the pulse durationcan be compensated by multiple reflection at dielectric mirrors having anegative group velocity dispersion. This is realized with the mirrors M4and M5 as shown in FIG. 5. For the use of a femtosecond laser/lasersystem according to the invention, in particular for a commercialapplication, a mounting of the elements of the laser system, inparticular the mirrors 2, 3 being part of the ring resonator as well asthe laser-active element 1 and all of the thereto belonging adjustmentmeans is advantageous performed on a common mounting platform. Inparticular the elements of the laser system may be integratedadvantageously in a monolithic metal block, which for example mayconsist of aluminium or also of invar.

FIG. 6 shows a laser resonator of a femtosecond laser according to asecond variant of the invention. Thereto, the embodiment shown in FIG. 6is supplied with an additional means for tuning the wavelength asdefined in claim 1. A preferred embodiment of the resonator shown heremay be established for instance by establishing the embodiment shown inFIG. 4 (according to a first variant of the invention—claim 7). Thebrewster-prism 3 according to claim 4 functions as an especialdispersive element and effectuates a spatial separation of distinctwavelengths. By tilting a resonator mirror, in particular of a mirror 43next neighboured to the prism, the wavelength may be selected for whichthe resonator shall have the least losses. This is to be the operationwavelength of the laser. Waves of shorter or longer wavelength are notcapable to be amplified in the resonator and get lost only after a fewcirculations.

FIG. 7 illustrates a further embodiment of the laser resonator of afemtosecond laser according to a second variant of the invention. Alsothe latter, establishing the embodiment shown in FIG. 3, may be suppliedwith an additional means for tuning the wavelength according to claim 1.Here the resonator comprises, in distinction to the embodiment shown inFIG. 6, as a special dispersive element half a Brewster-prism accordingto claim 5 whose adjacent leg 34 supports an antireflex-coating for thelaser wavelength. This is an especially advantageous embodiment as bythis way only half of the positive GVD compared to FIG. 6, which is tobe compensated by the dispersive mirror, is introduced into theresonator.

FIG. 8 serves to define the distance d between the concave mirrors 21,22 as well as the distances X1 and X2 of the laser-active element fromthe concave mirrors 21, 22.

FIG. 9 depicts the region of stability of a laser system according to afirst and a second variant of the invention having a 1 GHz resonator andconcave mirrors 21 and 22 with a focus—length of 15 mm. The variation ofthe beam waist radius w in a laser-active/self-focussing element isoutlined as a function of the average power P of the resonator mode.Thereby w is the beam waist radius in the laser-active element 1 and Pis the average power of the resonator. The portrayal is performed in atwo-dimensional way as a function of the mirror distance d on the onehand and the crystal position X1 on the other hand. A negative amount ofthe value w⁻¹∂w/∂P is advantageous to support the occurrence of a smoothaperture due to the Kerr-effect in the laser-active element. It isrecognized, that such circumstance applies if the mirror distance d isselected smaller than the sum of the focus lengths f21 and f22 of theconcave mirrors 21 and 22. It is further recognized that this effect isamplified if the distance X1 is selected smaller than half of the sum ofthe focus length.

The outstanding beam quality of the inventive femtosecond laser/lasersystem becomes clear from FIG. 10. The figure shows the distribution ofthe intensity perpendicular to the resonator plane measured with a CCDcamera. The full line is a numerical Gauβ-function fitted to themeasured values. The outstanding consistency is recognized. The insetshows the distribution of intensity of the total beam cross sectionmeasured by a CCD camera. The measured intensity is correlated withluminance values. It is recognized the almost circular beam profile of aTEM₀₀ mode.

The superb/beam quality of the inventive femtosecond laser shows also upin the envolvement of the beam radius w as a function of the distancefrom the outcouple mirror. This is illustrated in FIG. 11. The dotsrepresent the experimentally measured values, the full line images theenvolvement as predicted by theory, whereby the formalism of the realbeam propagation has been used by taking into account the so calledM²-factor. A superb consistency of theory and experiment is recognizedfor a M²-factor of 1,07, which is very tight to the theoretical optimumof M²=1,0.

FIG. 12 shows the generated pulsed laser power as a function of thepower of the pump laser for various degrees of transmission T of theoutput coupler 3. In case of a degree of transmission T of 1% a quantumefficiency η of 6,6% results. The latter increases at a degree ofoutcoupling T of 2% to a value of η=20%. The degree of outcoupling T=1%represents the smallest practically usable degree of outcoupling of theoutput coupler 3. In case of an inventive laser system as realized herewith a ring resonator, optimized to a pulse repetition frequency of 1GHz, the degree of outcoupling T=2% represents the experimentallydetermined optimum of degree of outcoupling T. Nevertheless also higherdegrees of outcoupling T may be suitable.

What is claimed is:
 1. Passive mode locked femtosecond laser having aring resonator comprising: a laser-active element having a groupvelocity dispersion; a prismatic element having a group velocitydispersion; at least one dielectric mirror, having a negative groupvelocity dispersion (GVD_(η)) for reflection; two concave mirrors whichare spatially arranged immediately adjacent to the laser-active elementand are oriented with their concave surfaces towards the laser-activeelement; and an optical output coupler, wherein the ring resonator hasan optical path and each of said laser-active element, prismatic elementand mirrors is arranged along the optical path, and wherein for acontiguous part of the optical spectrum capable of being amplified bythe laser-active element, the value of the negative group velocitydispersion associated with the at least one dielectric mirror is greaterthan the combined value of the positive group velocity dispersionassociated with the laser active element and the prismatic element,characterized in that the optical path of the resonator is below 120 cm,the focus length of each of the concave mirrors, which are arrangedadjacent next to the laser-active element is below 3 cm, whereby a highrepetition generation of femtosecond pulses is attainable and one of (i)said at least one dielectric mirror or (ii) the output coupler or (iii)a further mirror is tiltable at a tilt angle upon an axis wherein byadjusting the tilt angle in interplay with the spatial dispersion of theprismatic element, a wavelength is adjustable for which the resonatorsupports laser activity.
 2. The laser according to claim 1, wherein theoptical path length of the resonator is below 60 cm.
 3. The laseraccording to claim 1, wherein the optical path length of the resonatoris below 30 cm.
 4. The laser according to claim 1, wherein the opticalpath length of the resonator is below 15 cm.
 5. The laser according toclaim 1, wherein the focus length of each of the concave mirrors isbelow 2 cm.
 6. The laser according to claim 1, wherein the focus lengthof each of the concave mirrors is below 1.5 cm.
 7. The laser accordingto claim 1, characterized in that the laser-active element is alsoself-focusing element.
 8. The laser according to claim 1, characterizedin that the resonator is adapted in interplay with the non-linearelement, such that pulsed laser activity allows a higher energy outputfrom the laser-active element as compared to continuous laser activity.9. The laser according to claim 1, characterized in that the prismaticelement has a surface and a surface of exit adapted to have an opticalbeam incident on the prism surface under condition of minimal deflectionwherein the optical beam is incident on the prism surface at a Brewsterangle for the wavelength amplifiable by the laser-active medium andescapes at the Brewster angle from the surface of exit.
 10. The laseraccording to claim 1, characterized in that the prismatic element has aprism surface of exit adapted to have an optical beam incident on theprism surface at a Brewster angle for the wavelength amplifiable by thelaser-active medium, wherein the optical beam escapes substantially at aright angle from the surface of exit.
 11. The laser according to claim10, characterized in that an antireflection coating for the wavelengthamplifiable by the laser-active medium is applied to the surface ofexit.
 12. Passive mode locked femtosecond laser having a ring resonatorcomprising: a laser-active element having a group velocity dispersion;at least one dielectric mirror having a negative group velocitydispersion (GVD_(η)) for reflection; two concave mirrors which arespatially arranged immediately adjacent to the laser-active element andare oriented with their concave surfaces towards the laser-activeelement; and an optical output coupler; wherein the ring resonator hasan optical path and each of said laser-active element and mirrors isarranged along the optical path, and wherein for a contiguous part ofthe optical spectrum capable of being amplified by the laser-activeelement, the value of the negative group velocity dispersion associatedwith the at least one dielectric mirror is greater than the value of thepositive group velocity dispersion associated with the laser activeelement, characterized in that the optical path of the resonator isbelow 60 cm, and the focus length of each of the concave mirrors whichare arranged immediately adjacent to the laser-active element is below 3cm, whereby a high repetition generation of femtosecond pulses isattainable.
 13. The laser according to claim 12, wherein the opticalpath length of the resonator is below 30 cm.
 14. The laser according toclaim 12, wherein the optical path length of the resonator is below 15cm.
 15. The laser according to claim 12, wherein the focus length ofeach of the concave mirrors is below 2 cm.
 16. The laser according toclaim 12, wherein the focus length of each of the concave mirrors isbelow 1.5 cm.
 17. The laser according to claim 1 or 12, characterized inthat both focus lengths of the concave mirrors which are adjacent to thelaser-active elements are substantially equal in value.
 18. The laseraccording to claim 1 or 12, characterized in that the distance of theconcave mirrors to each other, which are adjacent to the laser-activeelement is smaller than the sum of their focus lengths.
 19. The laseraccording to claim 18, characterized in that the distance of thelaser-active element from a first concave mirror differs by more than 2%from the distance of the laser-active element from a second concavemirror.
 20. The laser according to claim 19, wherein the distancediffers by more than 50%.
 21. The laser according to claim 19, whereinthe distance differs by more than 10%.
 22. The laser according to claim1 or 12, characterized in that the distance of the concave mirrors toeach other which are adjacent to the laser-active element is greaterthan the sum of focus lengths of the concave mirrors, and a ringaperture is located in the ring resonator.
 23. The laser according toclaim 1 or 12, characterized in that the laser-active elementsubstantially comprises at least one of Ti:Saphir, Cr:LiSAF,Cr:Forsterite, Cr:LiSGaF, CR:LiCAF and Yb:YAG.
 24. The laser accordingto claim 1 or 12, comprising a frequency doubled Nd:YVO⁴⁻, Yb:YVO⁴⁻,Nd:YAG- or Yb:YAG-laser used as a pump laser.
 25. The laser according toclaim 1 or 12 characterized in that the degree of outcoupling of theoptical output coupler is below 5%.
 26. The laser according to claim 25,wherein the degree of outcoupling is below 3%.
 27. The laser accordingto claim 25, wherein the degree of outcoupling is below or equal to 2%.28. The laser according to claim 1 or 12, characterized in that theoutput coupler is configured as a partial reflecting mirror.
 29. Thelaser according to claim 1 or 12, characterized in that all mirrors havea negative group velocity dispersion (GVD).
 30. The laser according toclaim 1 or 12, characterized in that only one mirror has a negativegroup velocity dispersion (GVD).
 31. The laser according to claim 1 or12, characterized in that a ring resonator is compensated forastigmatism.
 32. The laser according to claim 1 or 12, characterized inthat all elements of a ring resonator are mechanically arranged on acommon mounting platform.
 33. The laser according to claim 1 or 12characterized in that two beams are reflected by each of the concavemirrors and an angle enclosed between the beaus amounts between 15° and25°.
 34. A method for obtaining a generation of femtosecond pulses fromand turning a passive mode locked femtosecond laser having a ringresonator comprising a laser-active element having a group velocitydispersion, a prismatic element having a group velocity dispersion, atleast one dielectric mirror having a negative group velocity dispersion(GVD_(η)) for reflection, two concave mirror which are spatiallyarranged immediately adjacent to the laser-active element and areoriented with respective concave surfaces towards the laser-activeelement, and an optical output coupler, wherein the ring resonator hasan optical path and each of said laser-active element, prismatic elementand mirrors is arranged along the optical path, and wherein for acontiguous part of the optical spectrum capable of being amplified bythe laser-active element, the value of the negative group velocitydispersion associated with the at least one dielectric mirror is greaterthan the combined value of the positive group velocity dispersionassociated with the laser active element and the prismatic element,characterized in that the optical path of the resonator is below 120 cm,the focus length of each of the concave mirrors, which are arrangedadjacent next to the laser-active element is below 3 cm, where in afirst step a high repetition generation of femtosecond pulses isobtained and in a second step one of (i) said at least one dielectricmirror or (ii) the output coupler or (iii) a further mirror is tiltableat a tilt angle upon an axis wherein by adjusting the tilt angle ininterplay with the spatial dispersion of the prismatic element, awavelength is adjusted for which the resonator supports laser activity.35. The method according to claim 34, characterized in that the passivemode coupled femtosecond laser is operated at a pulse repetition rateexceeding 500 MHz.
 36. The method according to claim 34, characterizedin that the passive mode femtosecond laser is operated at a pulserepetition rate exceeding 1 GHz.
 37. The method according to claim 34,characterized in that the passive mode femtosecond laser is operated ata pulse repetition rate exceeding 2 GHz.
 38. The method according toclaim 34, characterized in that the passive mode femtosecond laser isoperated at a pulse repetition rate exceeding 10 GHz.
 39. A method forobtaining a generation of femtosecond pulses from a passive mode lockedfemtosecond laser having a ring resonator comprising a laser-activeelement having a group velocity dispersion, at least one dielectricmirror having a negative group velocity dispersion (GVD_(η)) forreflection, two concave mirrors which are spatially arranged immediatelyadjacent to the laser-active element and are oriented with respectiveconcave surfaces towards the laser-active element and an optical outputcoupler, wherein the ring resonator has an optical path and each of saidlaser-active element and mirrors is arranged along the optical path, andwherein for a contiguous part of the optical spectrum capable of beingamplified by the laser-active element, the value of the negative groupvelocity dispersion associated with the at least one dielectric mirroris greater than the value of the positive group velocity dispersionassociated with the laser active element, characterized in that theoptical path of the resonator is below 60 cm, and the focus length ofeach of the concave mirrors which are arranged immediately adjacent tothe laser-active element is below 3 cm, where a high repetitiongeneration of femtosecond pulses is attained.
 40. The method accordingto claim 39, characterized in that the passive mode coupled femtosecondlaser is operated at a pulse repetition rate exceeding 500 MHz.
 41. Themethod according to claim 39, characterized in that the passive modefemtosecond laser is operated at a pulse repetition rate exceeding 1GHz.
 42. The method according to claim 39, characterized in that thepassive mode femtosecond laser is operated at a pulse repetition rateexceeding 2 GHz.
 43. The method according to claim 39, characterized inthat the passive mode femtosecond laser is operated at a pulserepetition rate exceeding 10 GHz.
 44. Passive mode coupled femtosecondlaser having a ring resonator comprising: a laser-active element havinga group velocity dispersion; two concave mirrors which are spatiallyarranged immediately adjacent to the laser-active element and areoriented with their concave surfaces towards the laser-active element;at least one dielectric mirror receiving a reflection from at least oneof said concave mirror, having a negative group velocity dispersion forreflection and wherein for a contiguous part of the optical spectrumcapable of being amplified by the laser-active element, the value of thenegative group velocity dispersion associated with the at least onedielectric mirror is greater than the value of the positive groupvelocity dispersion associated with the laser active element; and anoptical output coupler receiving a reflection from at least one of saidat least one concave mirror and dielectric mirror, whereby a highrepetition generation of femtosecond pulses is obtained.
 45. Passivemode coupled femtosecond laser having a ring resonator comprising: alaser-active element having a group velocity dispersion, two concavemirrors which are spatially arranged immediately adjacent to teelaser-active element and are oriented with their concave surfacestowards the laser-active element; at least one dielectric mirror, havinga negative group velocity dispersion for reflection and wherein for acontiguous part of the optical spectrum capable of being amplified bythe laser-active element, the value of the negative group velocitydispersion associated with the at least one dielectric mirror is greaterthan the value of the positive group velocity dispersion associated withthe laser active element; and an optical output coupler, wherein thering resonator has an optical path and each of said laser-activeelement, mirrors and coupler is arranged along the optical path, andwhereby a high repetition generation of femtosecond pulses is obtained.46. Passive mode coupled femtosecond laser having a ring resonatorcomprising: a laser-active element having a group velocity dispersion; aprismatic element having a group velocity dispersion, two concavemirrors which are spatially arranged immediately adjacent to thelaser-active element and are oriented with their concave surfacestowards the laser-active element; at least one dielectric mirror, havinga negative group velocity dispersion for reflection and wherein for acontiguous part of the optical spectrum capable of being amplified bythe laser-active element, the value of the negative group velocitydispersion associated with the at least one dielectric mirror is greaterthan the value of the positive group velocity dispersion associated withthe laser active element; and an optical output coupler, wherein thering resonator has an optical path and each of said laser-activeelement, mirrors and coupler is arranged along the optical path, andwhereby a high repetition generation of femtosecond pulses is obtained,and one of said at least one dielectric mirror, output coupler, or afurther mirror is tiltable at a tilt angle upon an axis wherein byadjusting the tilt angle in interplay with the spatial dispersion of theprismatic element, a wavelength is adjusted for which the resonatorsupports laser activity.
 47. Passive mode coupled femtosecond laserhaving a ring resonator comprising: a laser-active element having agroup velocity dispersion; two concave mirrors which are spatiallyarranged immediately adjacent to the laser-active element and areoriented with their concave surfaces towards the laser-active element;at least one dielectric moor receiving a reflection from at least one ofsaid concave mirror, and a prismatic element receiving a reflection fromthe at least one dielectric mirror, wherein the at least one dielectricmirror has a negative group velocity dispersion for reflection andwherein for a contiguous part of the optical spectrum capable of beingamplified by the laser-active element, the value of the negative groupvelocity dispersion associated with the at least one dielectric mirroris greater than the combined value of the positive group velocitydispersion associated with the laser active element and at least oneprismatic element; and an optical output coupler receiving a reflectionfrom at least of one of said concave mirror and at least one dielectricmirror, whereby a high repetition generation of femtosecond pulses isobtained, and one of said at least one dielectric mirror, outputcoupler, or a further mirror is tiltable at a tilt angle upon an axiswherein by adjusting the tilt angle in interplay with the spatialdispersion of the prismatic element, a wavelength is adjusted for whichthe resonator supports laser activity.